[llvm] [LoopInterchange] Don't consider loops with BTC=0 (PR #167113)

[Michael Kruse via llvm-commits]

================
@@ -602,14 +606,17 @@ struct LoopInterchange {
   DependenceInfo *DI = nullptr;
   DominatorTree *DT = nullptr;
   LoopStandardAnalysisResults *AR = nullptr;
-
   /// Interface to emit optimization remarks.
   OptimizationRemarkEmitter *ORE;
+  // A cache to avoid recalculating the backedge-taken count for a loop.
+  std::map<const Loop *, const SCEV *> LoopBTC;
----------------
Meinersbur wrote:

```suggestion

  // A cache to avoid recalculating the backedge-taken count for a loop.
  DenseMap<const Loop *, const SCEV *> LoopBTC;
```
Avoid `std::map` if not needed, it has *terrible* runtime and memory characteristics: no hashing but binary tree, one malloc per entry.

<img width="700" height="500" alt="rnw9R" src="https://github.com/user-attachments/assets/2f51c4f5-1cb6-4fd0-8a7c-60d8e2af865b" />

https://stackoverflow.com/questions/43191216/differences-similarities-between-llvmdensemap-and-stdmap

https://github.com/llvm/llvm-project/pull/167113